Conventionally, a technology for introducing fuel gas (evaporated fuel) evaporated in a fuel tank of a vehicle into a cylinder of an engine to prevent leakage of fuel components to the outside of the vehicle is known. Evaporated fuel in the fuel tank is temporarily recovered by a canister, and purge gas containing the evaporated fuel desorbed from the canister is introduced into an intake path. A purge valve for adjusting the flow rate of the purge gas is placed on a purge path for connecting the canister and the intake path, and the degree of opening of the purge valve is controlled in response to an operation state of the engine.
For example, in Patent Document 1 (Japanese Patent Laid-Open No. 2000-45886), a method for purging evaporated fuel absorbed to absorbent in the canister to an intake path of an engine is disclosed. In the technology, the evaporated fuel absorbed to the absorbent is vaporized by introducing a negative pressure of the intake path into the canister in a closed state with respect to the atmosphere, and the evaporated fuel vaporized in the canister is purged to the intake system by a difference between the pressure in the canister stepped up by the vaporization and the pressure in the intake path. The flow rate of the evaporated fuel purged to the intake path is grasped based on the magnitude of the pressure difference between the canister and the intake path and the magnitude of the absolute pressure in the canister.
It is to be noted that, in Patent Document 1, the canister is placed between the fuel tank in a sealed state and the intake path, and a vacuum control valve is placed between the fuel tank and the canister. The vacuum control valve is opened when the pressure in the fuel tank becomes higher than a predetermined pressure. Consequently, the evaporated fuel in the fuel tank is recovered by the canister, and the pressure in the fuel tank drops. Such purge of the evaporated fuel performed for the object of reduction of the pressure in the fuel tank as described above is referred to as high-pressure purge, reduced pressure purge or the like.
However, in the method disclosed in Patent Document 1 described above, it is necessary to acquire in advance a relationship between the magnitude of the pressure difference between the canister and the intake path and the flow rate of evaporated fuel to be purged in response to the magnitude of the absolute pressure in the canister. Further, it is necessary to store all of the acquired data in an electronic controlling apparatus. In addition, complicated working for acquiring all data is additionally performed. As a result, it is necessary to provide a ROM having a great capacity in the electronic controlling apparatus and there is the possibility that the cost may increase.
Further, in the high-pressure purge performed when the pressure in the fuel tank is high, the pressure on the upstream side of a valve for purge (purge valve) such as vacuum control valve as that in Patent Document 1 becomes higher than the atmospheric pressure. Therefore, where the degree of opening of the purge valve is controlled similarly as upon normal purge in which evaporated fuel recovered by the canister is purged, there is a high possibility that the flow rate of the purge gas may increase from an intended introduction ratio of purge gas.
That is, in the high-pressure purge, it is difficult to obtain an intended flow rate of purge gas, and there is the possibility that a rich air-fuel mixture may be introduced in the cylinder of the engine. Further, in such a case as just described, there is a concern that the control may be complicated in that the control for adjusting the amount of fuel to be injected from an injector is required separately and so forth. Accordingly, it is desired to introduce, also in the high-pressure purge, purge gas into the intake system with an intended introduction ratio of purge gas without complicated control.